1. Field of the Invention
The present invention relates generally to magnetic polymer microbeads, and a method for preparing the same, and in particular to the magnetic polymer microbeads prepared by swelling the polymer particles with a solvent, and then encapsulating the magnetic nanoparticles in the polymer particles, and a method for preparing the same
2. The Prior Arts
In recent years, the magnetic bioseparation technology has been widely applied in biological-related areas, such as the separation and purification of cells, DNA, RNA, antibodies, antigens, and proteins. The magnetic polymer microbeads are widely applied in the magnetic bioseparation for clinic testing, and function as the carriers for such as, the enzyme immobilization, bacterial separation, cell separation, and the separation of nucleic acid and protein. The magnetic polymer microbeads can also be used as the carriers of the medicines and gene delivery. In addition, the magnetic polymer microbeads can also be used as the magnetic carbon powders, magnetic inks, magnetic coatings, and the like.
In the application of the magnetic cell separation, the average particle size of the magnetic polymer microbeads used for antibody immobilization ranges from nanometers to microns. The magnetic polymer microbeads are now commercially available from (1) Dynal Biotech., Norway, known as Dynabeads® which are made of polystyrene, and have an average particle size of about 2.8 μm. The antibody-immobilized magnetic Dynabeads can selectively bind the specific cells, and thereafter, the isolation is effected by the magnetic field, and finally the linkage between the antibodies on the magnetic Dynabeads and the antigens on the cells is cleaved by the enzyme; (2) BD Bioscience, the United States, known as BD™Mag magnetic polymer microbeads which have a particle size ranging from nanometers to microns (about 0.1 to 0.45 μm). The magnetic polymer microbeads carrying monoclonal antibody can bind the specific cell, and followed by separation by BD Imagent, or magnetic separator; (3) Seradyn Inc., the United States, known as Sera-Mag™ beads which have an average particle size of about 1 μm; (4) Polysciences, Inc. the United States, known as BioMag® beads which have an average particle size of about 1 μm, and consist of an iron oxide core with a silane coating; (5) Polysciences, Inc. the United States, known as estapor® superparamagnetic microspheres which have an average particle size ranging from submicrons to microns.
The methods for preparing the above-mentioned magnetic polymer microbeads can generally be divided into two categories: (1) the magnetic material core is surrounded by the polymer coat; and (2) the magnetic materials are evenly dispersed within the polymer matrix, or are filled in the pores of the polymer matrix. Conventionally, in the case of the magnetic core material surrounded by a polymer, the magnetic metal nanoparticles are surrounded by silane coat to form the magnetic polymer microbeads with the average particle size of about 0.1-10 μm, which are mainly used in immunosorbant assays. However, the magnetic metal nanoparticles can also be surrounded by non-silane coat. For example, the U.S. Pat. No. 4,267,234 disclosed that a magnetic metal nanoparticle was surrounded by a polyglutaraldehyde coat, and the aldehyde groups on the polymer can readily be used to immobilize the proteins, such as antibodies, or antigens. The U.S. Pat. No. 4,454,234 disclosed that acrylamide, n-butylacrylate, N,N′-methylenebisacrylamide, and the magnetic particles of LaMn2Ge2 were mixed together, and polymerized at 37° C. to form the submicron-sized magnetic polymer microbeads. Then, the magnetic polymer microbeads were modified, wherein the ester groups on the polymer were converted to carboxyl groups, or acid amide groups, and which were used to bond to proteins. U.S. Pat. No. 4,554,088 disclosed that the magnetic polymer microbeads were prepared by surrounding Fe3O4 core with a silane coat, and then chemically modified using diazotization, carbodiimide or glutaraldehyde. Such magnetic polymer microbeads were applied in bioseparation because they could couple with the antibodies for uses in bioseparation. U.S. Pat. No. 4,783,336 disclosed that the magnetic polymer microbeads were prepared by surrounding a magnetic Fe3O4 core with a polyacrolein coat. U.S. Pat. No. 6,204,033 disclosed that the magnetic polymer microbeads were prepared by surrounding a ferromagnetic or superparamagnetic material with a polyvinyl alcohol coat. U.S. Pat. Nos. 4,452,773 and 4,795,698 disclosed that the magnetic polymer microbeads were prepared by coating a magnetic Fe3O4 core with a natural polymer (such as dextran) and bovine serum albumin (BSA), respectively. All the above-mentioned patents disclosed that the magnetic polymer microbeads were prepared by surrounding magnetic particles with polymer coats so that the magnetic particles were used as core of the magnetic polymer microbeads, and the polymers having the functional groups were used as coat. However, although the magnetic polymer microbeads could be prepared according to the above-mentioned methods, the particle shapes and sizes of the magnetic polymer microbeads are not easily controlled, and the distribution of the particle diameters of the magnetic polymer microbeads is relatively broad, and the magnetic polymer microbeads look irregular, and tend to agglomerate.
In order to solve the above-mentioned problems, other methods involving evenly dispersing the magnetic metal materials within the polymer matrix were provided. In such methods, the monomers and the magnetic material were mixed together, and then polymerized to obtain the magnetic polymer microbeads. For example, U.S. Pat. No. 4,358,388 disclosed that the magnetic polymer microbeads are prepared by the suspension polymerization (oil in water). In this method, the monomers, the magnetic metal nanoparticles (Fe3O4), the initiator, and the solvent were mixed together, emulsified and suspended in an organic phase, and then poured into a water phase to undergo the polymerization, and finally the magnetic polymer microbeads in which the magnetic nanoparticles are evenly dispersed were obtained. In addition to the method of polymer coating on magnetic core, magnetic polymer microbeads can be made by a converse manner, i.e., forming a layer of magnetic material on polymer particles. U.S. Pat. No. 5,320,944 disclosed that the magnetic polymer microbeads used for immunoassay were obtained by coating the polystyrene particle with iron, cobalt, or nickel oxide type magnetic material. U.S. Pat. No. 5,091,206 disclosed that the metal oxide Fe3O4 was mixed with monomers and coated onto the polystyrene core, and then the polystyrene core was surrounded by another polymer formed after polymerizing the monomers, and thereby the surface of the new formed particle had the Fe3O4 magnetic materials. Moreover, the method for preparing the aforementioned Sera-Mag™ particles is another typical method. For example, U.S. Pat. No. 5,648,124 disclosed that the magnetic polymer microbeads were prepared by filling the magnetic material in the pores of the pre-prepared spherical polymer particles with a narrow distribution of particle sizes. Similarly, U.S. Pat. No. 4,654,267 disclosed that a porous polymer microparticles were prepared from the monomers of methyl methacrylate, glycidyl methacrylate, and the like, and then the porous polymer microparticles were evenly mixed with iron(II, III) salts (which could be used to prepare Fe3O4) or the material for making other magnetic particles, and then ammonium hydroxide solution was added therein, and the mixture was heated so that the Fe3O4 particles could be made within the pores of the polymer. However, the amount of the magnetic nanoparticle, incorporated in the porous polymer microparticls prepared from various monomers using such a method, was varied with the monomers used in the polymerization, and the magnetic nanoparticle content in the particles was found to be about 5-20%.
In the above U.S. Pat. No. 4,654,267, the magnetic polymer microbeads having spherical shape, narrow particle size distribution, and good quality were obtained. However, the method has its limitation because the magnetic materials need to be prepared in situ within the pores of the polymer particles. For example, in order to produce the magnetic particles, the precursor of the magnetic material and the reaction reagent have to diffuse into the pores of the polymer particle, and contact with each other in the pores of the polymer. In such a method, the magnetization of the magnetic polymer microbeads cannot reach to a much higher level because the choice of the magnetic materials is limited, and the total amount of the magnetic material incorporated in the magnetic polymer microbead (the magnetic moment per unit volume of particle) have their limit.